This invention relates to a fluidized bed heat exchanger and a method of generating steam, and, more particularly to such a generator and method in which a plurality of fluidized beds are provided for generating heat.
Fluidized beds are well recognized as attractive heat sources since they enjoy the advantages of an improved heat transfer rate, while permitting a reduction in corrosion, boiler fouling, and sulfur dioxide emission. In a typical fluidized bed arrangement, air is passed upwardly through a mass of particulate material causing the material to expand and take on a suspended or fluidized state. However, there is an inherent limitation on the range of heat input to the water passing in a heat exchange relation to the fluidized bed, largely due to the fact that the quantity of air supplied to the bed must be sufficient to maintain same in a fluidized condition yet must not cause excessive quantities of the particulate material to be blown away.
This disadvantage is largely overcome by the heat exchanger disclosed in U. S. Pat. No. 3,823,693 issued to Bryers and Shenker on July 16, 1974, and assigned to the same assignee as the present application. In the arrangement disclosed in the latter patent, the furnace section of the heat exchanger is formed by a plurality of vertically stacked chambers, or cells, each containing a fluidized bed. The fluid to be heated is passed upwardly through the fluidized beds in a heat exchange relation thereto to gradually raise the heat content of the fluid. A tube bundle is located in the area above each bed to provide a convection surface for the effluent gases from each bed. The particulate material is separated from the effluent gases exiting from the beds and recycled back into the lowermost bed which functions as a recycle cell to burn off the remaining carbon in the particulate material.
However, the fraction of flow area available above each bed to receive the tube bundles is relatively small due to limitations placed on the cross-sectional area of each cell caused by tube spacings, welding accessibility, combustion requirements, etc. As a result, the convection surface defined by the tube bundles is limited to an extent that the mass flow of the effluent gases per area of convection surface and the resulting heat transfer coefficient above each bed, is less than optimum. Also since the recycle bed is not provided with fresh fuel, undesirable variations in heat input due to fuel variations or steam generator output changes are often encountered. Further, there is no provision to control the inventory and the fluidizing velocity in the recycle cell, which further adds to the problems of controlling the heat in the beds.